The defamation trial of a British activist began Tuesday in Bangkok after he co-authored a report alleging widespread labour abuses in Thailand's food industry, a prime supplier of cheap foodstuffs to Western supermarkets.

Andy Hall, 34, faces jail if convicted over lawsuits brought by Thai fruit processor Natural Fruit, after a report he wrote levelled accusations of forced and child labour, unlawfully low wages and long hours at one of its factories.

Natural Fruit is a major supplier to the European drink market, and the migrant rights campaigner has accused the company of trying to divert attention from the findings through its legal action.

Leading European food giants have called for the charges to be dropped, while rights groups have condemned the defamation proceedings from a Thai food industry that has faced a battery of bad headlines over recent months.

Prosecution witnesses are expected to begin testimony on Tuesday as the trial, which is linked to an interview Hall gave to the Al-Jazeera television network, gets under way.

Defamation is a criminal offence in Thailand and Hall, whose passport has been confiscated by Thai authorities pending the trial, could be jailed for up to a year if found guilty.

More serious charges under the computer crime act -- which carries up to seven years in jail for each count -- are due to be heard later in September.

Natural Fruit is also seeking $10 million damages through a civil suit.